The term "asphalt" or "asphalt cement" as used in the description of the present application refers to any of a variety of solid or semi-solid materials at room temperature which gradually liquify when heated, and in which the predominant constituents are naturally occurring bitumens or which are obtained as residue in petroleum refining. Asphalt is further defined by Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 3, Third Ed. (1978) pp. 284-327, John Wiley & Sons, New York. An additional discussion appears in the publication entitled "A Brief Introduction to Asphalt and Some of its Uses", Manual Series No. 5 (MS-5), The Asphalt Institute, 7th Ed., September, 1974. Both of these references are hereby incorporated by reference.
Asphalt cements have found particular utility when combined with aggregates. Such combinations, generally referred to as "asphalt concrete", are employed extensively as paving compositions for roads, driveways, parking lots, airport runways, etc. The asphalt is converted to a fluid state when paving a road. One fluid form is the suspension or emulsion of the asphalt in water. After spreading and compressing the aggregate-containing asphalt, water evaporates or separates, and the asphalt hardens into a continuous mass. Another form of asphalt employed in road construction is a cutback, i.e., a liquid petroleum product produced by fluxing an asphaltic base with a suitable organic solvent or distillate. A road is formed by paving the aggregate-containing cutback and evaporating the volatile distillate from the mass. An advantage of using the above road construction techniques is the avoidance of high temperature application. In an alternative and most widely used technique, the asphalt and aggregate can be mixed and applied at elevated temperatures at the fluid state of the asphalt to form the road. This form of asphalt, which is neither cut-back nor emulsified generally is referred to as asphalt cement.
The degree and rate of hardening of asphalt cement during application and while in service ("age hardening") are factors affecting the durability of a surface such as a road pavement. A certain amount of hardening of a freshly applied surface is often desirable in order to allow the newly placed surface to be placed into service quickly. However, excessive hardening and loss of ductility of an asphalt based surface can dramatically reduce its useful lifetime. Januszke, in "Industrial Engineering Chemistry Product Research and Development", Vol. 10, (1971), 209-213, indicates that lead and zinc diethyldithiocarbamates were effective in inhibiting the adverse hardening.
In certain geographic areas, such as desert regions in the western and southwestern United States, deterioration of an asphalt road may occur quickly and is often extensive. Embrittlement and cracking of the road surface often results.
Because of the high demand for better quality materials for roofs and for the pavement of roads, airfields and other applications, there have been many suggestions in the art for producing improved asphaltic compositions.
It is known in the art that excessive age hardening of paving asphalts can be reduced through the use of certain antioxidants such as lead or zinc dithiocarbamates. The above-mentioned reference by Januszke discusses an evaluation of the effect of 24 antioxidants on paving asphalt durability and is incorporated by reference for its teachings regarding the problem and methods of assessing the performance of additives.
It also has been suggested, for example, that the properties of asphalt can be improved by incorporating into the asphalt, small amounts of polymeric materials. U.S. Pat. No. 2,486,487 describes potting and sealing compounds useful for impregnating coils and other electrical equipment, and these compositions comprise asphalts, synthetic waxes of the cetyl acetamide type and solid polymers of normal butyl methacrylate, particularly those having a softening point of about 65.5.degree.-74.degree. C. It is stated that the incorporation of the polymer results in a marked increase in the viscosity, and the blend exhibits good adhesion, flexibility and cold temperature resistance. Thermoplastic terpene hydrocarbon resins can be used in place of the polymers of butyl methacrylate.
A number of patents have described the modification of the properties of asphalt mixtures by including therein, copolymers and interpolymers of alkyl acrylates and alkyl methacrylates with olefinic monomers such as ethylene (U.S. Pat. Nos. 2,972,588; 3,414,533; 3,980,598; 4,492,781; and 4,511,689).
U.S. Pat. No. 3,567,476 describes a process for coloring bituminous materials. In the process, a resinous substance such as a latex and a pigment are mixed with the bituminous emulsions. A resinous substance and pigment coat the individual masked colloidal particles of bituminous material. A large number of suitable resinous substances are disclosed in this patent, and included among the substances are acrylic polymer latices.
U.S. Pat. No. 3,951,895 describes asphaltic compounds for use in paving and roofing in which an acrylic emulsion of a copolymer of methyl methacrylate and ethylacrylate is mixed with 1 to 3 parts of hydrated lime and 40 parts of sand. This acrylic emulsion then is blended with an asphalt emulsion.
U.S. Pat. No. 4,018,730 describes methods for emulsifying asphalt-rubber paving materials. The rubber materials which may be included in the asphalt paving material include a wide variety of natural as well as synthetic rubbers including acrylic rubber.
U.S. Pat. No. 4,222,916 describes coal tar emulsions containing sufficient acrylate emulsions to provide a desired viscosity. Sand may be added to the compositions. The acrylate emulsion is exemplified by butyl acrylate emulsion.